1. Field of the Invention
The present invention relates to CPP (current-perpendicular-to-the-plane) magnetic detecting elements, and particularly to a CPP magnetic detecting element having a structure which CIP (current-in-the-plane) magnetic detecting elements do not allow and in which the magnetization of a pinned magnetic layer is more firmly fixed by the uniaxial anisotropy of the pinned magnetic layer.
2. Description of the Related Art
A magnetic detecting element having a multilayer composite including a free magnetic layer, a nonmagnetic material layer, and a pinned magnetic layer may be of a CIP type or a CPP type, according to the direction of current flowing in the multilayer composite.
In a CIP magnetic detecting element, current flows in a direction parallel to the surfaces of the layers of the multilayer composite. In a CPP magnetic detecting element, current flows in the direction perpendicular to the surfaces of those layers.
The CPP magnetic detecting element generally has an advantage that the size can be reduced to increase the reproduction power in comparison with the CIP magnetic detecting element. It is therefore believed that the CPP magnetic detecting element has a structure capable of achieving a high density recording, and that it can be used in place of the currently dominating CIP magnetic detecting element.
In order to put the CPP magnetic detecting element into practical use to achieve a high density recording, the rate of change in resistance (ΔR/R) is desirably increased. Japanese Unexamined Patent Application Publication No. 2002-92826 has taught that a multilayer structure defining a pinned layer P or a free layer F, constituted of a magnetic layer FF and a nonmagnetic layer FN produces an electron scattering effect, that is, the resistance at the interface between the magnetic layer FF and the nonmagnetic layer FN has large spin dependency and increases the CPP magnetoresistance (CPP-MR) (see, for example, paragraph [0048] in the specification of this publication).
FIG. 10 of this publication shows a multilayer structure constituted of a first nonmagnetic layer SN1 and a second nonmagnetic layer SN2, defining a nonmagnetic interlayer S disposed between the pinned layer P and the free layer F.
Japanese Unexamined Patent Application Publication No. 8-7235 has disclosed a technique for fixing the magnetization of a pinned magnetic layer by the uniaxial anisotropy of the pinned magnetic layer. The disadvantages of this technique will be described later.
Known CPP magnetic detecting elements, including the CPP magnetic detecting element of Japanese Unexamined Patent Application Publication No. 2002-92826, have the following disadvantages.
FIG. 10 is a schematic diagram of the structure of a known CPP magnetic detecting element. The CPP magnetic detecting element includes a multilayer composite and electrodes 5 and 6 disposed on the top and the bottom of the multilayer composite. The multilayer composite includes nonmagnetic material layers 2, pinned magnetic layers 3, and antiferromagnetic layers 4 disposed in that order over and under a free magnetic layer 1.
In this structure, each pinned magnetic layer 3 is composed of three sublayers: two magnetic layers 3a and 3c and a nonmagnetic interlayer 3b between the magnetic layers 3a and 3c. The magnetic layers 3a and 3c are magnetized antiparallel to each other. Such a multilayer structure is called the artificial ferrimagnetic structure.
The antiferromagnetic layer 4 has a specific resistance as high as, for example, about 200 μΩ·cm2 (or more), and generates Joule heat when a current is applied to the electrodes 5 and 6. The Joule heat causes lattice vibration of conduction electrons in the adjacent pinned magnetic layer 3, the nonmagnetic material layer 2, and the free magnetic layer 1, thereby making the phonon scattering and electromigration vigorous.
It is considered that the rate of change in resistance (ΔR/R) of the CPP magnetic detecting element is closely related to the spin-dependent bulk scattering effect. In the structure shown in FIG. 10, the ΔR/R value depends on the magnetic layer 3c adjoining the nonmagnetic material layer 2 among the free magnetic layer 1 and the layers constituting the pinned magnetic layer 3. In order to increase the ΔR/R value, it is necessary that the difference in spin diffusion length between the up-spin conduction electrons and down-spin conduction electrons in the magnetic layer 3c be increased by setting the spin-dependent bulk scattering coefficient β of the magnetic layer 3c to be positive so that, in the magnetic layer 3c, the up-spin conduction electrons can easily flow while the down-spin conduction electrons can be easily scattered.
However, it has been found that the above-mentioned lattice vibration of conduction electrons causes phonon scattering to scatter the conduction electrons independently of the spin states, and that consequently the GMR effect represented by the ΔR/R value cannot be appropriately enhanced.
Furthermore, in the structure shown in FIG. 10, the presence of the thick antiferromagnetic layers 4 increases the gap between the electrodes 5 and 6. This makes it impossible to appropriately increase the recording density (more specifically, track recording density) of recording media.
One approach for enhancing the GMR effect of the CPP magnetic detecting element is to eliminate the antiferromagnetic layers 4 from the multilayer composite. In this instance, the magnetization of the pinned magnetic layers needs to be appropriately fixed without the antiferromagnetic layers.
The above-cited Japanese Unexamined Patent Application Publication No. 8-7235 has disclosed a technique for fixing the magnetization of a pinned magnetic layer by the uniaxial anisotropy of the pinned magnetic layer, without an antiferromagnetic layer.
However, the magnetic detecting element of this publication is of a CIP type, and there is no mention of how the magnetization of the pinned magnetic layer is fixed in a CPP magnetic detecting element. In addition, the pinned ferromagnetic layer (pinned magnetic layer) of this publication is deposited on a buffer layer made of tantalum, which is liable to turn amorphous and whose specific resistance is high. If such a buffer layer is used in a CPP magnetic detecting element, it probably generates heat as the known antiferromagnetic layer does, thereby causing conduction electrons to scatter independently of the spin states. Hence, the GMR effect cannot be enhanced. Furthermore, Japanese Unexamined Patent Application Publication No. 8-7235 has not clearly disclosed the principle of how the tantalum buffer layer firmly fixes the magnetization of the pinned ferromagnetic layer. Therefore, the structure of this publication cannot be directly applied for the structure of the CPP magnetic detecting element.
Probably, the rate of change in resistance (ΔR/R) of the CIP magnetic detecting element is closely related to the spin-dependent interface scattering, unlike the ΔR/R value of the CPP magnetic detecting element. If the structure of the interface between the nonmagnetic material layer and the pinned magnetic layer is changed in the CIP magnetic detecting element, there is a high risk of reducing the ΔR/R value. It should be avoided to change the interface structure. In general, a nonmagnetic material layer is formed of Cu and a pinned magnetic layer is formed of a CoFe alloy or the like. Since the Cu/CoFe interface produces an excellent spin-dependent interface scattering effect, it is impractical that, for example, another layer is disposed between the Cu layer and the CoFe layer to modify the Cu/CoFe interface structure of the CIP magnetic detecting element.
In the CPP magnetic detecting element, on the other hand, the ΔR/R value is, probably, related to the spin-dependent bulk scattering rather than the spin-dependent interface scattering. The inventors of the present invention have thought that the CPP magnetic detecting element allows of modification of the interface structure between the nonmagnetic material layer 2 and the pinned magnetic layer 3 (magnetic layer 3c) shown in FIG. 10 for more firmly fixing the magnetization of the pinned magnetic layer 3, unlike the CIP magnetic detecting element.